Conventionally, as this kind of combustion apparatus, there is known an apparatus in Patent Document 1 in which, in order to improve the heat resistivity of the combustion box, a water jacket as a cooling means for cooling the combustion box is disposed in that portion of the combustion box which lies between the burner and the heat exchanger. The water jacket is constituted by: a laterally elongated first jacket part disposed on one of the front-side and the rear-side side plates of the combustion box; a laterally elongated third jacket part disposed on the other of the front-side and the rear-side side plates of the combustion box; a second jacket part which is elongated in the front side and the rear side and which is disposed on laterally one-side side plate of the combustion box, and which connects the first jacket part and the third jacket part together; an inlet-side jacket part which is disposed in a portion closer to one of the front-side and the rear-side side plates of laterally the other-side side plate of the combustion box and which introduces the water passed through the heat exchanger to the first jacket part; and an outlet-side jacket part which is disposed in a portion closer to the front-side and the rear-side side-plates of the combustion box and into which the water from the third jacket part flows. A hot water outlet port is provided, at an upper part thereof, with an outlet-side jacket part.
It is thus so arranged that the water passed through the heat exchanger flows from the inlet-side jacket part through the first jacket part, the second jacket part, and the third jacket part, to the outlet-side jacket part, and thus water (hot water) can be supplied from the hot-water outlet port to the hot water passage which is connected to the hot-water outlet port. Further, in this example, a fourth jacket part is disposed in a manner to be elongated in longitudinally one side from an upper part of the outlet-side jacket part toward an inlet-side jacket part. That portion of laterally the other-side (the other-side) side plate of the combustion box which lies between the inlet-side jacket part and the outlet-side jacket part is arranged to be cooled by the fourth jacket part.
By the way, there are cases where a part of the heat absorbing tubes is locally overheated (boiled) and, consequently, gives rise to generation of air bubbles in the heat absorbing tubes of the heat exchanger. These air bubbles are carried by the water flow to thereby flow down to the outlet-side jacket part. In this connection, conventionally, the fourth jacket part is elongated horizontally from an upper part of the outlet-side jacket part toward longitudinally one side. In addition, the sectional center of the tip closer to the inlet-side jacket part of the fourth jacket part is positioned at a height equivalent to the sectional center of hot water outlet port. Therefore, if the air bubbles flow into the fourth jacket part without being discharged out of the outlet-side jacket part, the air bubbles are likely to get accumulated in the fourth jacket part. Then, since that portion of the fourth jacket part in which the air bubbles get accumulated can no longer be cooled by water, the temperature therein will increase and consequently the scales are likely to get accumulated, resulting in deterioration of the durability.